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Arthropod Borne Diseases, by A.B.S.
Recent attention on the West Nile Encephalitis outbreak has brought
the need for more education and awareness to the threat of insects transmitting
diseases to both humans and the livestock we use for food. Unfortunately
most people are fixated on the West Nile problem and continue to ignore the
myriad of other pathogens that can be transmitted by insects and other arthropods
in North America. Most of the pathogens that are transmitted by arthropod
vectors are of low virulence and due to exposure most of us have developed
an immunity to them long ago. Unfortunately, with the migration of much of
the population into areas of the country that had until recently been wilderness,
the increasing introduction of new species of both pathogens and vectors
into native ecosystem, and an increase in global temperatures, that is changing
much of the habitat, an ever increasing number of cases are being seen in
hospitals nation-wide. In this article we will discuss just a few of these
diseases.
Plague
Many well known insect borne diseases occur in North America, but in such small
numbers that most of the population is blissfully unaware anything is happening.
One such pathogen is Plague. Better known as The Black Plague this bacterial
disease swept through Europe and Asia during the middle ages killing an estimated
34 million people. While such large outbreaks are no longer a significant threat,
several small outbreaks and single cases do appear annually. Most commonly
transmitted by the bite of an infected flea, usually the Oriental Rat Flea.
The largest known reservoir of Plague in North America is found in Prairie
Dog towns around the Four Corners region. With 10-15 human cases in the U.S.
and up to 3,000 human cases worldwide annually, Plague can manifest itself
in any of three different forms.
Pneumonic plague occurs when Y. pestis infects the lungs. This type of plague
can spread from person to person through the air. Transmission can take place
if someone breathes in aerosolized bacteria, which could happen in a bioterrorist
attack. Pneumonic plague is also spread by breathing in Y. pestis suspended
in respiratory droplets from a person (or animal) with pneumonic plague. Becoming
infected in this way usually requires direct and close contact with the ill
person or animal. Pneumonic plague may also occur if a person with bubonic
or septicemic plague is untreated and the bacteria spread to the lungs.
Bubonic plague is the most common form of plague. This occurs when an infected
flea bites a person or when materials contaminated with Y. pestis enter through
a break in a person's skin. Patients develop swollen, tender lymph glands (called
buboes) and fever, headache, chills, and weakness. Bubonic plague does not
spread from person to person.
Septicemic plague occurs when plague bacteria multiply in the blood. It can
be a complication of pneumonic or bubonic plague or it can occur by itself.
When it occurs alone, it is caused in the same ways as bubonic plague; however,
buboes do not develop. Patients have fever, chills, prostration, abdominal
pain, shock, and bleeding into skin and other organs. Septicemic plague does
not spread from person to person.
Mosquito Borne Diseases
The majority of the U.S. population is totally unaware of the majority of insect
transmitted diseases. While West Nile is now a common household term, the more
commonly occurring Jamestown Canyon, LaCrosse, St. Louis, Western Equine, Eastern
Equine, and Venezuelan Encephalitis have never been heard of by most. These
mosquito transmitted viruses are seen every year in the U.S., but due to the
fact that most of the cases are asymptomatic or have very mild symptomology,
coupled with the fact that most cases are in rural, often economically poor
settings, these diseases get very little press. Though most cases are mild,
acute cases begin with flu like symptoms that then can progress to inflammation
of the brain. This can lead to coma or even death. Survivors of acute encephalitis
often suffer varying degrees of brain damage.
Also receiving little press is the increasing occurrence of malaria that originates
U.S. nationals that have picked up the disease while overseas and from infected
persons that have recently migrated to the U.S. Several of these individuals
have entered the country illegally, and therefore bypass needed healthcare
due to fear of capture. This scenario is also being observed with several other
diseases.
Dengue Fever also known as "break bone fever" is another mosquito transmitted
disease that is of concern. Two versions of the disease can occur, with one
version being a hemorrhagic fever. Dengue fever usually starts suddenly with
a high fever, rash, severe headache, pain behind the eyes, and muscle and joint
pain. The severity of the joint pain explains the name "breakbone fever".
Nausea, vomiting, and loss of appetite are common. A rash usually appears 3
to 4 days after the start of the fever. The illness can last up to 10 days,
but complete recovery can take as long as a month. Older children and adults
are usually sicker than young children.
Most dengue infections result in relatively mild illness, but some can progress
to dengue hemorrhagic fever. With dengue hemorrhagic fever, the blood vessels
start to leak and cause bleeding from the nose, mouth, and gums. Bruising can
be a sign of bleeding inside the body. Without prompt treatment, the blood
vessels can collapse, causing shock (dengue shock syndrome). Dengue hemorrhagic
fever is fatal in about 5 percent of cases, mostly among children and young
adults.
Chagas
One disease that is becoming more of a concern among my fellow entomologists
is Chagas Disease also known as American Trypanosomiasis. This disease is transmitted
by the >Kissing Bug=, which blood feeds on the human victims as they sleep,
usually biting the host on the face. The bugs are found in houses made from
materials such as mud, adobe, straw, and palm thatch in Central and South America.
During the day, the bugs hide in crevices in the walls and roofs. During the
night, when the inhabitants are sleeping, the bugs emerge. After they bite
and ingest blood, they defecate on the person. The person can become infected
if T. cruzi parasites in the bug feces enter the body through mucous membranes
or breaks in the skin. The unsuspecting, sleeping person may accidentally scratch
or rub the feces into the bite wound, eyes, or mouth.
The parasite causes damage to the cells of the heart. It is estimated that
as many as 18 million people in Mexico, Central America, and South America
have Chagas disease, most of whom do not know they are infected. Another 500,000
people in the U.S., many of whom are illegal aliens, are thought to have the
disease. Since the insects that transmit this disease are also found throughout
much of the U.S. the possibility of transmission does exist.
Though not common, human cases of Chagas are appearing more often in the U.S.
with many more veterinary cases in dog and raccoon populations. These animal
populations then act as a reservoir for the parasite. While it is commonly
thought that better housing found in the U.S. will prevent widespread rates
of infection, this is not entirely accurate. A recent infection of an infant
that occurred in central Tennessee happened when the insect entered the patients’ home.
The infants mother happened to observe the insect on the child, and having
just watched a broadcast about Chagas on television, recognized the insect.
The mother requested a parasite screening by the family physician that isolated
the organism. The child was then able to get proper medical care. A check of
wildlife and pet populations in the area around the residence found high infection
levels among the animals. If the mother hadn't seen the insect, the child could
have been infected for years without proper treatment. Transmission can occur
when someone is bitten on camping trips as well as in the home. The disease
can also be transmitted through blood transfusions, organ transplants, to fetus
during pregnancy, and by eating feces contaminated food.
Ehrlichiosis
The presence of Rocky Mountain Spotted Fever, Lyme Disease, and Tick Fever
is fairly well known by many people throughout the U.S. Unfortunately these
tick transmitted diseases are just the tip of the iceberg.
One tick borne disease becoming increasingly common is Ehrlichiosis. Human
ehrlichiosis due to Ehrlichia chaffeensis was first described in 1987.
The disease occurs
primarily in the southeastern and south central regions of the country. To
date six species of bacteria are known to cause Human Ehrlichiosis and are
transmitted by three know tick vectors, the Lone Star Tick, Black Legged Tick,
and the Western Black Legged Tick. Most victims of Human Ehrlichiosis have
had underlying immunosuppressants, but this isn't always the case.
In my home town in Tennessee an outbreak of Ehrlichiosis was detected in one
of our more well know retirement communities. Researchers, including myself
found that the majority of the victims had been exposed to the disease through
tick bites that occurred while they were playing golf. The victims were generally
older, but had been considered in good health. Patients with ehrlichiosis generally
visit a physician in their first week of illness, following an incubation of
about 5-10 days after the tick bite. Initial symptoms generally include fever,
headache, malaise, and muscle aches. Other signs and symptoms may include nausea,
vomiting, diarrhea, cough, joint pains, confusion, and occasionally rash. Ehrlichiosis
can be a severe illness, especially if untreated, and as many as half of all
patients require hospitalization.
Prevention
The best way to beat these diseases is to prevent the initial infection. This
is often easier said than done. Several things can be done to limit your
risk of exposure.
- Wear long sleeved shirts when possible
- Tuck pants into boots or socks
- Use repellents such as DEET (N,N-Diethyl-meta-Toluamide) or Permethrin
(Note: Permethrin should not be applied to the skin, only to clothing. Studies
have shown that DEET products containing more than 7% DEET should not be applied
to the skin.
- Wear light colored clothing when possible to aid spotting ticks
- Self check thoroughly when you return indoors
- Keep screens on windows and doors in good repair
- Caulk cracks and other entry point on homes
- Use mosquito netting when camping. Information on these and other arthropod
transmitted diseases can by found through the US
Centers for Disease Control web site.
With the proper education and a little preparation, the risk from insect transmitted
diseases can be greatly reduced.
